This is an update to my previous post. Maybe I'm wrong about this and both my Thunderbird, FM webmail, and the other places I tested just have the 7-bit ASCII limitation.
I looked at the raw text on gmail and FM's webmail. Both showed,
I guess =92 is the right curved quote because hex 92 is what I see in the
extended ASCII table so I guess FM doesn't change it. It just can't display it.
My Thunderbird's raw text for this however looks like,
Code:
I can=EF=BF=BDt think o
which I am not sure what that is. 3-byte Unicode maybe for the black diamond with a question mark in it?